Polymethyl methacrylate (PMMA) is a material valued for its excellent optical properties (in particular the gloss and a high transparency with a transmission of visible light of at least 90%). However, it is also a brittle thermoplastic material which is sensitive to impacts. This characteristic is related to the fact that the glass transition temperature of PMMA is approximately 110° C., so that, in this material, the polymer chains are not capable of easily moving at ambient temperature. For some applications, it is therefore necessary to improve the impact strengthening of PMMA while retaining its transparency.
The impact strengthening of PMMA is generally improved by virtue of the introduction into the acrylic resin of an impact modifier of the type which is known as core-shell, which exists in the form of multilayer spherical particles. These particles are prepared by emulsion polymerization and are recovered in the powder form by atomization. They comprise a sequence of “hard” and “soft” layers. It is thus possible to encounter bilayer (soft-hard) or trilayer (hard-soft-hard) particles. In the case of cast acrylic sheets, obtained by polymerization of the mixture of monomers in a mould, the impact modifier is dispersed beforehand in the mixture of monomers. In the case of extruded acrylic sheets, the impact modifier is compounded in the extruder with the acrylic resin. In both cases, it is necessary for the impact modifier to be well dispersed in the acrylic resin in order to maintain a constant and homogeneous level of impact strength.
Application WO 01/57133 of the Applicant Company discloses a methacrylic (co)polymer strengthened by an impact modifier and by a grafted elastomeric copolymer. The impact modifier can be an additive of core-shell type or a block copolymer comprising at least one block obtained from a diene or from an alkyl or aralkyl (meth)acrylate. The grafted elastomeric copolymer is obtained from an elastomeric copolymer to which methacrylic (co)polymer groups are grafted in the pendent position. The impact strengthening thus results from the combination of two polymers, the impact modifier and the grafted elastomeric copolymer.
International Application WO 99/29772 discloses the impact strengthening of semi-crystalline thermoplastic resins using a styrene-butadiene-methyl methacrylate block copolymer (SBM).
International Application WO 02/055573 of the Applicant Company discloses the impact strengthening of a PMMA using a block copolymer of ABA type in which B denotes a central block obtained from a diene, for example an SBM.
International Application Wo 03/062293 of the Applicant Company discloses a process for the impact strengthening of a thermoplastic matrix using a B(-A)n block copolymer composed of a central block B and of n branches A and prepared using the controlled radical polymerization technique. This process applies to the strengthening of numerous thermoplastics (PS, PC, PVDF, and the like) and in particular to the manufacture of cast PMMA sheets.
According to this process, in a 1st stage, the central block B is prepared using a polyfunctional alkoxyamine. In a 2nd stage, the central block B is mixed with the monomer(s) intended to form the branches A, which results in the formation of the B(-A)n block copolymer. In this stage, a radical initiator can be added to the mixture, which results in the formation of a matrix. In a 3rd stage, the block copolymer B(-A)n, optionally mixed with the matrix, is separated from the residual monomers by evaporation under vacuum at temperatures ranging up to 250° C. (stage referred to as desolventization stage). In a 4th stage, the block copolymer, thus freed from the residual monomers, can subsequently be extruded with a thermoplastic resin or else redissolved in a mixture of monomers, which is itself subsequently polymerized. On conclusion of this 4th stage, a block copolymer dispersed in a matrix is thus obtained.
The process of WO 03/062293, applied to the manufacture of cast sheets, is not capable of transfer to the industrial scale. This is because it exhibits the disadvantage of requiring a stage of desolventization, followed by a stage of redissolution of the copolymer. First, these two unit operations, by increasing the overall cycle time, affect the output of the process. Secondly, the desolventization stage is also capable of resulting in the formation of gels in the block copolymer, which affects its redissolution in the mixture of monomers and, consequently, can damage the transparency of the cast sheet.
Furthermore, according to the process disclosed, in particular in the examples, it is preferable, during the 2nd stage, to initiate the formation of the branches A at the same time as that of the matrix. For this, the monomer A is brought into contact with two types of initiators, the conventional radical initiator and the reactivatable central block. The monomer A is thus consumed at the same time according to two competing radical polymerization mechanisms, each exhibiting specific kinetics. The control of this 2nd stage is very difficult as it implies the harmonization of the rates of formation of the blocks A and of the matrix. This implies that it is necessary to adjust the nature of the radical initiator to the central block B and thus also to carefully adjust the temperature cycle. In practice, contradictory requirements are encountered and the possible compromises generally result:                in premature phase separation during the polymerization of the copolymer B(-A)n, which migrates to the interface of the sheet and mould. In this case, sheets which are impossible to remove from the mould and/or which are partially or completely opaque are obtained;        in unacceptable contents of residual methyl methacrylate (MMA), which it is impossible to remove once the sheet is finished.        
The Applicant Company has now improved the process for the preparation of impact-strengthened cast acrylic sheets disclosed in International Application WO 03/062293. The cycle time of this process is improved with respect to that disclosed in WO 03/062293 as it does not require any desolventization-redissolution stage. The process of the invention thus exhibits an improved productive output.
Furthermore, the radical initiator is added after and not during the formation of the block copolymer B(-A)n, which facilitates the control of the polymerization and, consequently, makes it possible to avoid the formation of defects at the surface of the sheet, the formation of opaque regions due to the phase separation of the copolymer B(-A)n and the presence of an unacceptable amount of residual MMA.
The Applicant Company has also found, with surprise, that a very good transparency/impact strength compromise is obtained if the proportion of the central block B in the sheet is between 2 and 5%, preferably between 2.5 and 4.5%, more advantageously still between 2.6 and 4.0%.
According to an alternative form of the invention, the manufacture of cast sheets can also be envisaged starting from a block copolymer B(-A)n preformed elsewhere. For example, it is possible to envisage, for example for reasons of cost or logistics, preparing the copolymer on a production site other than that of the cast sheets, optionally even by another manufacturer. The process then comprises the following stages:    1. mixing the block copolymer B(-A)n with MMA and optionally with at least one comonomer M and with at least one radical initiator;    2. casting the mixture obtained in stage 1 in a mould and then heating it in order to obtain a cast sheet.
Preferably, the proportion by weight of the central block B in the sheet is between 2 and 5%, preferably between 2.5 and 4.5%, more advantageously still between 2.6 and 4.0%.
Furthermore, the block copolymer has a tendency to settle down inside the matrix to give homogeneously distributed particles. The particles exist in the form of substantially spherical nodules inside which one or more subnodule(s), having the same composition as the MMA homo- or copolymer, are present.